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It is often refreshing and surprisingly useful
to research topics that you had previously
overlooked. In this issue gas discharges come
under the spotlight. We include papers that
describe how gas discharges and plasmas may
be analysed, and also a paper (refer to p. 56)
that shows how gas discharges (sparks) can be
used for generating very high energy acoustic
pulses for use underwater for sub-bottom
profiling. We also see how algorithms derived
from the world of image processing have been
used for removing noise from electric
discharge data (refer to p. 41) and how this
same filtering procedure can be applied to a
very wide range of data acquisition problems.
Every now and then a new area of
technology opens up and you immediately
know that it is destined to have a profound
impact. Lasers are one example and they in
turn have led to the development of the
entirely new field of Terahertz or T-ray
imaging (refer to p. 20). ‘‘Terahertz’’ refers to
a band in the electromagnetic spectrum that
lies between infrared and radio. Just as X-rays
have a shorter wavelength than visible light,
T-rays wavelengths are longer. This area of
the spectrum has always been there of course,
but until recently it has been difficult and very
expensive to actually generate T-rays, and
so they have remained very much in
a technology backwater.

Lasers are key to this new development as
T-rays are produced by firing a near-IR laser
pulse at a semiconductor optical switching
device. Like X-rays they pass through some
materials easily but are attenuated by others
and can therefore be used much like X-rays
for medical and security applications.
It is always useful to have a variety of tools
at your disposals and one major advantage
that T-rays have over X-rays is that being nonionising
they are far less hazardous to use.
Getting images beyond the visible spectrum
is also the topic of ourTutorialwhich deals with
the very latest developments in infrared
imaging. The early days of pyroelectric
vidicons are now giving way to very exotic
semiconductors and ferroelectric devices.
Not only are the resolutions available from
such devices increasing all the time but also the
temperatures at which they can operate are
getting closer to ambient levels. Devices that
had previous required liquid nitrogen cooling
can now be operated at 110 K which can be
achieved with low cost, and much more user
friendly, thermoelectric cooling. In the future it
is hoped that operation at 190 K will be
possible.

In much the same way that I do not believe
anyone would have anticipated a fraction of
the applications that have been found
for lasers. I also suspect that the future will
have much the same to say about Terahertz
imaging.